This invention relates in general to video formats for displaying information on a raster scan display device, and in particular to a video output format compiler and programmable video output formatter that can be programmed to produce different video formats to meet different display requirements.
Video formats were developed as methods of displaying picture element (or pixel) information on a raster display device such as a cathode-ray tube (CRT). The picture on a CRT screen is "painted" by an electron beam that is accelerated toward the phosphor coated screen by a high positive voltage near the face of the tube. When the electrons hit the screen, the phosphor emits visible light. The electron beam makes a horizontal scan by moving along a horizontal line across the screen from left to right, and makes a vertical scan by incrementally moving down in the vertical direction until it reaches the bottom of the screen. As the beam completes all the horizontal scans in a single vertical scan, a frame is displayed. Because the phosphor's light output decays exponentially with time, the entire picture (or frame) must be refreshed (redrawn) many times per second so that the viewer sees what appears to be a constant, unflickering picture. This is referred to as the vertical field rate.
Video display devices are characterized by a particular scan rate, which specifies the number of scanning lines per frame, and the vertical field rate. A display device is further characterized by a specified horizontal resolution which is the number of pixels per line, and vertical resolution which is the number of lines per raster scan frame. The timing relationship between these signals is governed by a synchronization method which is a complex combination of horizontal and vertical timing (pulse) information that controls the decoding of the video signal by a display device or other video processing equipment.
These video signal specifications are generally referred to as a video format. A video format represents a periodic sequence of a time-varying signal (or signals) that defines the method by which pixel information has been encoded, horizontal and vertical resolution and synchronization method.
Based on the operating requirements of traditional television equipment, a set of standards defining video formats were developed. However, new and varied applications for display devices requiring different scan rates and resolutions soon brought deviations from those standards. As a result, over the years, many different standards have been developed (e.g. NTSC, PAL, RS-343 and RGB), with each employing different scan rates.
The variety in display devices that require different video formats has led to interfacing problems between different display devices and existing systems. For example, for a computer to connect to different monitors, it's video output format must be modified to meet the requirements of the target monitor.
At present, changing the video output format of a given processing system must be done either by modifying the hardware, or through a complex set of hardware-specific instructions. The hardware-only solution requires an alteration of hardware to change the video output format, which in some instances may necessitate as much as a new display generator. The second approach requires an expert knowledge of the operation of the hardware to develop a complex and tedious description of the format. Thus, both approaches tend to be very inefficient and costly.
There is, therefore, a need for a more efficient solution to the problem of generating multiple video formats for a single processing system that may connect to several different display devices.